The present invention relates to a radio communication device, more particularly, for example, a microwave-band radio receiver for receiving signals of broadcast waves wirelessly transmitted in microwave bands, a microwave-band radio communication system and electronic equipment.
Conventional microwave-band radio communication systems include those having radio-frequency radio transmitters and radio-frequency radio receivers as shown in FIG. 21 (see, e.g., JP 2003-258655 A). Herein, microwave bands refer to frequency bandwidths including millimeter-wavebands.
First, description is given of a conventional radio-frequency radio transmitter 1900 shown in FIG. 21. The radio-frequency radio transmitter 1900 is composed of a first up-converter (intermediate frequency mixer) section 1300, a second up-converter (microwave-band mixer) section 1350 and a transmission antenna 1100.
The first up-converter section 1300 has a local oscillator 1200, a frequency mixer 1400, a filter 1500, an amplifier 1600, a power combiner 1801 and an attenuator (level setter) 1850.
In this conventional radio-frequency radio transmitter 1900, at first, the first up-converter section 1300 receives inputs of a first IF signal (modulated wave signal) IF1. The modulated wave signal IF1 is a signal modulated by, for example, orthogonal multi-carrier modulation (OFDM modulation). The frequency mixer 1400 multiplies an inputted signal IF1 and a first LO signal (local oscillation signal) LO1 (frequency fLO1) together to output a second IF signal IF2 (frequency fIF2). Further, the filter 1500 provided immediately after the mixer 1400 mainly extracts only a second IF signal IF2 component. In this description, the filter 1500 passes upper sideband signals as desired signals.
Moreover, a power divider 1802 distributes part of the first local oscillation signal LO1 outputted by the local oscillator 1200 to the attenuator 1850, which adjusts the level of the distributed first local oscillation signal LO1, and then the power combiner 1801 combines the first local oscillation signal LO1 and the second IF signal IF2 (frequency fIF2).
Thus, a multiwave signal (intermediate frequency multiple signal) composed of the second IF signal IF2 (radio-frequency signal RF1 (frequency fRF1)) and the first local oscillation signal (reference signal) LO1 is formed. This intermediate frequency multiwave signal is amplified by the amplifier 1600, and the amplified intermediate frequency multiwave signal is inputted into the second up-converter section 1350.
Description is now given of the second up-converter section 1350. The second up-converter section 1350 has a second local oscillator 1250, a mixer 1700, a filter 900 and an amplifier 1000.
The mixer 1700 multiplies a multiwave signal composed of the second IF signal IF2 and the first local oscillation signal LO1 by a second LO signal (local oscillation signal) LO2 (frequency fLO2) outputted from the local oscillator 1250 to up-convert the signal to a radio signal. Further, the filter 900 passes only desired frequency components of the up-converted radio signal, and the amplifier 1000 amplifies the passed radio signal. The signal amplified by the amplifier 1000 is a multiple signal composed of a signal of a radio modulated signal component (radio-frequency signal RF1+second LO signal LO2) and a signal of a local oscillation signal component (first local oscillation signal LO1+second local oscillation signal LO2).
The transmission antenna 1100 transmits a signal of a radio modulated signal component (frequency fRF1+frequency fLO2) and a signal of a local oscillation signal component (frequency fLO1+frequency fLO2).
Description is now given of a conventional radio-frequency radio receiver 2900. The radio-frequency radio receiver 2900 is composed of a receiving antenna 2100, a first down-converter section 2300 and a second down-converter section 2350.
The radio-frequency radio receiver 2900 receives signals transmitted by the radio-frequency radio transmitter 1900. As schematically shown in a portion indicated by reference symbol Z in FIG. 21, signal components transmitted by the radio frequency radio transmitter 1900 are composed of a radio reference signal of a local oscillation signal component (frequency fLO1+frequency fLO2) and a signal of a radio modulated signal component (frequency fRF1+frequency fLO2). The radio-frequency radio receiver 2900 down-converts the received signals to original modulated wave signals (first IF signal IF1).
The first down-converter section 2300 has a filter 2500, an amplifier 2000, a mixer 2700 and a local oscillator 2200. The signal received by the receiving antenna 2100, which is a signal component 3000 transmitted by the radio-frequency radio transmitter 1900, is composed of a signal of a local oscillation signal component (frequency fLO1+fLO2) and a signal of a radio modulated signal component (frequency fRF1+fLO2). The filter 2500 passes a necessary signal out of the signal frequency (fLO1+fLO2) and the signal (frequency fRF1+fLO2), and the signal passed the filter 2500 is amplified by the amplifier 2000. With a local oscillation signal LO2 (frequency fLO2) from the local oscillator 2200, first frequency conversion is performed in the frequency mixer 2700.
Thus, the signal of the local oscillation signal component (frequency fLO1+fLO2) and the signal of the radio modulated signal component (frequency fRF1+fLO2) are down-converted to an intermediate frequency multiple signal (i.e., a multi-wave signal composed of a second IF signal IF2 and a first local oscillation signal LO1).
Then, after being amplified by an amplifier 2600, the multi-wave signal (frequency fRF1+fLO1) is inputted into the second down-converter section 2350 and divided by a divider 2800, and through a filter 2950 passing only a radio-frequency signal RF1 (frequency fRF1), the radio-frequency signal RF1 (frequency fRF1) is inputted into a frequency mixer 2400. In the meanwhile, the first local oscillation signal LO1 goes through a filter 2930 which passes only the first local oscillation signal LO1 and is amplified by an amplifier 2650 before being inputted into the frequency mixer 2400. The frequency mixer 2400 multiplies the radio-frequency signal RF1 by the first local oscillation signal LO1 so that the radio-frequency signal RF1 is down-converted and demodulated to a first IF signal IF1.
The conventional radio-frequency radio receiver 2900 has following problems.
A multi-wave signal, which is the signal down-converted in the first down-converter section 2300, contains a second IF signal IF2 (radio-frequency signal RF1) and a first local oscillation signal LO1.
In the first up-conversion step on the side of the transmitter 1900, the radio-frequency signal RF1 uses, for example, upper sidebands (frequency fLO1 of first local oscillation signal LO1+frequency fIF1 of first IF signal IF1). Therefore, the radio-frequency signal RF1 as a component of the multi-wave signal outputted after the first down-conversion on the side of the receiver 2900 contains a component of (frequency fIF1+frequency fLO1).
However, an output signal from the first down-converter section 2300 as a receiving side contains a signal of (frequency fLO1−frequency fIF1). This is mainly caused by the following reasons.
That is, the bandpass filter 1500 on the side of the transmitter 1900 is imperfect, and so at the time of first frequency up-conversion on the side of the transmitter 1900, the radio-frequency signal RF1 contains not only a component of (frequency fLO1+frequency fIF1) but also a component of (frequency fLO1−frequency IF1). At the time of the second up-conversion in the transmitter 1900, a frequency component (fLO1−fIF1) is up-converted and strengthened by the transmission amplifier 1000. Consequently, on the transmitter 1900 side, the undesired wave of the frequency [(fLO1−fIF1)+fLO2] is generated although its signal level is sufficiently low.
On the receiver 2900 side, in the first down-converter section 2300, the first down-conversion is performed with the second LO signal LO2 outputted by the local oscillator 2200, as a result of which a component of (frequency fLO1±frequency IF1) is generated as a component of the radio-frequency signal RF1. In addition, in the case where a radio transmission distance is relatively short, the peak level of the frequency component (fLO1+fLO2) that is a local oscillation signal component is larger by about 20 dB than the radio modulated signal component (fRF1+fLO2), and particularly, the nonlinearity in the intermediate frequency amplifier 2600 in an intermediate frequency stage is strong, by which the frequency component (fLO1−fIF1) that is an undesired wave is strengthened.
Thus, once an undesired wave component is amplified in the intermediate frequency amplifier 2600 in the first down-converter section 2300 on the receiver 2900 side and the undesired wave component is strengthened by non-linear action, complete filtration becomes difficult even though desired signal waves are separated in the filter 2950 and the filter 2930 in the subsequent stages.
For example, an undesired wave component that is the frequency component (fLO1−fIF1) remains though its level is smaller than that of (fLO1+fIF1) side of the desired wave, becomes larger than noise floor of the receiver 2900, and turns out to be a signal having phase distortion and noise added thereto. Consequently, during frequency down-conversion in the second down-converter section 2350 in the next stage, the frequency component (fLO1−fIF1) that is a desired wave functions as a noise component.
During the second down-conversion, the frequency component (fLO1+fIF1) that is a desired wave is converted to the frequency band identical to the frequency component of the undesired wave. Consequently, compared to a CN (Carrier/Noise ratio) value as signal quality of an input signal on the transmitter 1900 side, a CN value of an output signal after the final down conversion on the receiver 2900 side is deteriorated.
In addition, a multi-wave signal amplified in the amplifier 2600 and inputted into the second down-converter section 2350 is divided in the divider 2800 and a radio-frequency signal RF1 (frequency fRF1) goes through the filter 2950 passing only the radio-frequency signal RF1 and is inputted into the frequency mixer 2400. At the same time, a first local oscillation signal LO1 goes through the filter 2930 passing only the signal of frequency fLO1 and is amplified by the amplifier 2650 before being inputted into the mixer 2400. In such configuration, when the frequency fRF1 and the frequency fLO1 are close, it is difficult to fulfill a filter which can pass only the component of the frequency fRF1.
In addition, since the filter 2950 for passing the frequency fRF1 allows desired signals to pass, the filter 2950 is a broadband filter compared to the filter 2039 which passes only the frequency fLO1 and is therefore composed of a plurality of resonators and the like having wide frequency ranges. Because of this, in a loop L1 composed of the divider 2800, the filer 2930, the amplifier 2650, the mixer 2400 and the filter 2950, a resonator composed of the amplifier 2650 and the filter 2950 constitutes a positive feedback loop. Eventually, with the resonator formed by the filter 2950, the loop L1 operates as an oscillator to generate undesired oscillation waves, causing a problem that it is difficult for the present configuration to normally demodulate desired signals.
In addition, the broadband filter 2950 is large in size and so the loop L1 is also large so that the frequency range of the positive feedback loop covers a wider region toward both the high and low frequency sides, thereby increasing unnecessary noise floor and constituting an unstable loop.
In addition, in the case of such configuration as automatically regenerating the local oscillation signal LO1 during the second down-conversion, when a lot of out-of-band undesired components are present and undesired wave components spread to wide bands in the first frequency down-conversion, it becomes difficult to completely extract a non-modulated signal only with a narrow-band filter 2930. Consequently, the local oscillation signal inputted into the mixer 2400 contains undesired wave components, which hinders normal second frequency down-conversion, thereby causing a problem of considerable deterioration in characteristics.